1. Field of the Invention
This invention relates to an oriented, multilayered film comprising at least one layer of an ethylene based polymer. More specifically, this invention relates to a multilayer film made wherein at least one layer is ethylene based random copolymer, and wherein the film has been coextruded prior to being stretched for orientation.
2. Description of the Prior Art
Oriented, multilayered films are an important class of industrial products useful principally in the wrapping or packaging of various products in commerce. In particular, oriented isotactic polypropylene film is known to be useful for its high barrier properties, clarity and stiffness. Polypropylene for oriented films is typically isotactic homopolymer, or are modified polypropylene blends of isotactic polypropylene with differing isotactic polypropylene, e.g., differing melt flow rates (MFR) or tacticity, atatic polypropylene, syndiotactic polypropylene, random polypropylene copolymers having minor amounts of ethylene or higher xcex1- olefins, and ethylene copolymers, see for example, U.S. Pat. No. 4,950,720 and WO 96/02386. As indicated in U.S. Pat. No. 4,921,749 and related art, other components such as petroleum resins may be added as well to modify properties such as heat sealing performance, gas permeability and stiffness. Such oriented polypropylene, particularly biaxially oriented polypropylene (BOPP), has many desirable properties. However, excellent hot tack and heat seal are not among these since such films lack significant heat sealing and hot tack performance at operating temperatures suited to efficient industrial practice.
WO 92/14784 suggests heat sealable blend compositions said to be suitable for film and film structures that include 30-70wt. % of a low melting polymer that may be an ethylene based copolymer having a density from 0.88 g/cm3 to 0.915 g/cm3 (g) MWD no greater than 3.5 and a composition distribution breadth index greater than 70 percent and (b) from (a) from 70 to 30 wt. % of a propylene based polymer having 88 mole % to 100 mole % propylene and from 12 mole % to 0 mole % of an alpha olefin other than propylene.
WO A 95/13321 suggests heat sealed articles and heat scalable films comprising a polymer blend of a first polymer having a narrow molecular weight and composition distribution and a second polymer having a broad molecular weight distribution and composition distribution.
Accordingly, various polymeric layers have been used or proposed as heat sealing layers for biaxially oriented polypropylene film. Among those used industrially and taught in the patent literature are polyolefins, ethylene-based and propylene-based. Preferred heat scaling layers for BOPP have in the past typically been high propylene-content polyolefins, e.g., propylene-ethylene copolymers with 1-20 wt % ethylene, and propylene-ethylene-butene terpolymers with up to about 10 wt % each of ethylene and butene and blends of those with other olefin polymers. See for example, U.S. patents 4,643,945 ) LLDPE with propylene terpolymer) and 4,921,749 (propylene-ethylene copolymers) and the others listed above. These polymer selections do reduce the chemical dissimilarity of the sealing layer and core layers so as to allow good adhesion between them, but result in the requirement that subsequent heat sealing be conducted at temperatures at or above 120xc2x0 C., requiring long contact time with heating elements or high heat input to achieve the necessary temperature ranges.
The invention has an oriented multilayer film comprising at least one outer layer comprising from 50 to 100 wt. % of said outer layer an ethylene copolymer having a density of 0.900-0.935 g/cm3 and a CDBI of 50-95% said outer layer being in contact with a polypropylene core layer, and said film having been made by coextrusion of said ethylene copolymer and said polypropylene layer and subsequent orientation. Preferably the ethylene copolymer outer or skin layer, or layers, will have a MWD (Mw/Mn) of 1.8-3.5 and a MI of 0.5-10 (ASTM D1238, 190xc2x0 C., 2.16 kg). The coextruded layers can be biaxially oriented on tenter frame equipment without difficulties caused by the presence of low molecular weight amorphous polymer fractions inherently present in traditional Ziegler-Natta linear low density polyethylene. Problems of undesirable adherence to drums and jaws in processing and the presence of extractables in the films made hereby are essentially eliminated. Optical properties of biaxially oriented polypropylene films (BOPP) are retained while the scaling temperature is lowered and the heat seal strength and the hot tack properties are improved. The ethylene copolymer heat seal layer thus offers a practical, economical solution for preparing improved heat seal BOPP for all of stiff, flexible and heat shrinkable packaging.
Contemplated is an oriented multilayer film comprising at least one outer layer comprising an ethylene copolymer having a density of 0.900-0.935 g/cm3 and a composition distributed breadth index (CDBI) of 50-95% constituting 50 to 100 wt. % of the outer layer, and the outer layer being in contact with a polypropylene core layer, the film being a coextrusion of said ethylene copolymer and the polypropylene layer and subsequent orientation. Wherein the film is biaxially oriented, the ethylene copolymer having a density of 0.910-0.935 g/cm3 , a molecular weight distribution (MWD) of 1.8-3.5, and a melt index (MI) determined according to ASTM D 1238 of 0.5-10. Wherein the ethylene copolymer has a density of 0.915-0.925 g/cm3. Wherein the outer layer is a polyethylene blend composition comprising 50-99 wt % of the ethylene copolymer and 1-50 wt % essentially compatible ethylene-based homopolymers or copolymers comprising ethylene and one or more of C3-C12 xcex1-olefins, cyclic olefins, vinyl aromatic and polar vinyl monomers, the blend composition having an overall density of from 0.910 to 0.925 and an melt index (MI) determined according to ASTM D 1238 of from 0.7 to 5.0 Wherein the essentially compatible ethylene-based copolymer constitutes up to 30 wt. % of the total blend and is an ethylene copolymer comprising one or more C4-C8 xcex1-olefin having a density of from 0.880 to 0.915 g/cm3. Wherein the polypropylene layer consists essentially of an isotactic polypropylene homopolymer or a polypropylene random copolymer. The film may comprise two of the outer layers and at least one polypropylene layer in contact with at least one of the outer layers. The second outer layer may be polypropylene.